Prepregs comprising fibers combined with a matrix resin are one form of prepreg. Conventional prepreg consists of hundreds or thousands of fibers embedded in a continuous mass of matrix. Reinforcing fibers can be glass fiber, carbon fiber, or many other types. The reinforcing fibers typically used are available commercially in continuous form in bundles known as tows, which vary widely in number of fibers per tow. Many matrix resins are available; however two kinds of matrix resin systems dominate the prior art: thermoplastic and partially cured thermoset polymers.
Thermoplastic polymers have advantages over thermosetting materials in fracture toughness, impact strength and environmental resistance. Thermoplastics also provide prepregs with indefinite shelf life, give the fabricator better quality assurance and avoid the storage and refrigeration problems associated with thermosetting prepreg. The disadvantage of thermoplastic polymers as a matrix material is the difficulty of uniformly coating the fibers due to the high viscosity of the molten polymer. Thermoplastic prepregs also typically are rigid and less well-suited for weaving or braiding and the resulting fabrics are stiff. Similarly, the rigidity of thermoplastic impregnated prepregs complicates the formation of complex shapes; heat must be focused at the point of contact to achieve conformability during layup.
Prepregs containing thermosetting pre-polymers, although relatively flexible, may be tacky, thus requiring a protective release coating, typically a release paper or film, which must be removed prior to use. While thermoset prepregs are acceptable for many applications, their tackiness and the requirement of a protective release coating have made thermoset prepregs unfeasible for weaving and braiding.
Continuous fiber prepregs can be produced by a number of impregnation methods including hot melt, solution, emulsion, slurry, surface polymerization, fiber comingling, film interleaving, electroplating, and dry powder techniques.
In hot melt processing, impregnation can be accomplished by forcing the fiber and resin through a die at high temperature under conditions that create high shear rates. This process completely encapsulates substantially all the fibers making the prepreg very stiff and brittle. Other disadvantages of this process include the high stress applied to the fibers and difficulties in impregnating the fiber tows with thermoplastics, leading to low processing speeds.
In solution coating, the matrix material is dissolved in solvent and the fiber is passed through this solution and then dried to evaporate the solvent. Two disadvantages of this process are that thermoplastics usually exhibit limited solubility at high concentration, and most engineering thermoplastics cannot be dissolved in a low boiling solvent at room temperature. Additionally, high solution viscosity results in the same impregnation problems as with hot melt, as well as causing the fibers to stick together. Another problem is the difficulty in removing the solvent. Further, traces of solvent left in the prepreg lead to undesirable porosity in the composite structures.
An emulsion process is one way to apply particulate polymer matrix material with a very small particle size to prepreg fiber by synthesizing the resin as an aqueous emulsion with a surfactant. The problem with this process is that the removal of the surfactant from the final prepreg is difficult.
Slurry coating or wet powder processing is a non-solvent coating technique designed to resolve the problem of the insolubility of most thermoplastics in a solvent at room temperature. In slurry coating, the powder is suspended in a liquid medium, wherein no solvency exists between the resin and the medium, and the fibers are drawn through the slurry. The slurried particulate matrix does not substantially wetout the fiber, resulting in the need for higher pressures to consolidate the matrix and fibers into a prepreg. This prepreg can be tacky, which is not suitable for weaving or braiding. Other disadvantages include the necessity for the removal of the liquid medium, volatiles, and dispersants or surfactants which are used to form the polymer/liquid colloidal state, the likelihood of aggregates in the slurry caused by poor mixing, and the possibility that polymer particles will settle during processing.
To achieve intimate mixing in emulsion or slurry coating, the particulate size of the slurry or emulsion should be smaller than the fiber diameter. For many of the thermoplastics that cannot be made by emulsion or dispersion polymerization, it is extremely difficult to produce such fine powder. Thus, a coarse blend between fibers and particles is obtained. The quality of the blend decreases as the particle size increases, leading to poor matrix distribution in the consolidated prepreg, and a poor composite structure.
In fiber comingling, the polymeric matrix is introduced in fibrous form. Polymeric and reinforcing fibers are mingled as dry blends. Effective impregnation depends on the degree of randomness of the intermingling of the resin and fiber throughout the system. Since no wetting of the reinforcing fibers by the matrix material occurs, higher pressures are needed to consolidate the prepreg under equivalent processing times and temperatures, as compared to completely wetted prepregs. Another disadvantage of comingling products is its higher bulk factor making it more difficult to fit in complex molds.
Film casting is one method for producing prepreg which resolves some of the problems associated with hot melt impregnation of thermoplastics. It consists of stacking a film layer of matrix material cast from either hot melt, or solution over the prepreg fibers. The fibers sandwiched between two films are heated and calendared to force the resin into the fibers. The resulting prepreg is a rigid sheet that is difficult to form into complex shapes without elaborate thermoforming techniques.
Powder coating of fibers coats the tows with a powdered resin using a dry electrostatic process and fusing the resin to the tow with high powered infrared ovens The powdered resin must be solid at ambient and elevated storage temperatures, and be capable of melting to permit flow and to penetrate the fiber tow when heated. Dry powder coating has a disadvantage of precise metered resin control. Another disadvantage of powder coating is shedding of resin from the tow before the high temperature fusing, making a poor quality prepreg. Another disadvantage is the resin must be ground into powder of specific size for optimum coating. The grinding process is expensive and makes this process more expensive.
Intermediate composite products, such as prepreg, must contain sufficient matrix, typically over 15% by volume, to permit consolidation of the components into a substantially void-free prepreg structure without requiring the incorporation of more matrix material. Linear prepregs can be converted into two and three dimensional product forms by weaving, braiding, filament winding, and other known processes. Alternatively, these prepregs can be used to create a discontinuous fiber reinforced feedstock for molding by chopping, cutting, or like known processes.
Prepreg can be converted to a preform of a predetermined shape and fiber orientation. Preforms can be produced by any one of the conventional textile preform making methods, such as weaving, braiding and knitting, or by processes such as filament winding and tape placement. Preforms ultimately or concurrently can be consolidated into composite parts by applying heat and pressure.
A powder coating process, such as the one disclosed in U.S. Pat. No. 5,756,206 to Davies et al. typically involves the following four steps;
Forming un-spread tow in various cross sections;
Coating un-spread tow with resin particles;
Partially melting the particles onto the surface of the un-spread tow; and
Taking up resulting towpreg onto bobbins.
During the melting process the resin particles which are placed on the surface of a fiber bundle, melt in discontinuous patches along the fibers, the only penetration of the fiber bundles is by capillary action and is minimal. The discontinuous patches portions of the underlying fiber to be exposed. This feature provides the reduced rigidity but exposes the fiber to damaging textile processes. The lumpy surface also increases bulk which requires deeper cavities than molds with lower bulk. The deeper cavity molds have more mass and require more heat input to reach melting temperatures of the matrix resin. This slows the process and takes longer to make finished articles. The lumpy surface can catch on fiber guide in a weaving, braiding operation, or any operation using the towpreg that requires pulling the tow through alignment guides or adjacent fibers to place the tow. The catching action can damage the towpreg or strip the resin from the surface of the tow. The damaged tow and missing resin areas of the resulting preform would produce an inferior composite article.
Therefore, a need exists in the industry for a relatively low bulk and flexible prepreg which can be accomplished by utilizing low pressure impregnation techniques and the advantages thereof.